The Integumentary System

Overview and Importance

The integumentary system is the body's largest organ system, serving as the primary barrier between the internal environment and the external world. It is essential for protection, sensation, temperature regulation, and more. Understanding this system is crucial for evaluating and treating skin injuries and diseases.

• Main components: Skin, sweat (sudoriferous) glands, oil (sebaceous) glands, hair, nails, and subcutaneous tissue.

• Clinical relevance: Knowledge of the integumentary system aids in the diagnosis and management of burns, infections, and skin cancers.

Structure of the Skin

Layers of the Skin

The skin consists of two main layers and an associated subcutaneous layer:

• Epidermis: Outermost, avascular, composed of keratinized stratified squamous epithelium.

• Dermis: Underlies the epidermis, vascular, mainly dense connective tissue.

• Subcutaneous tissue (Hypodermis): Not part of the skin proper, but anchors skin to underlying structures, provides insulation and shock absorption (mainly adipose tissue).

The Epidermis

The epidermis is a keratinized, stratified squamous epithelium with four main cell types and four or five distinct layers (strata):

• Keratinocytes: Produce keratin, the protein that gives skin its protective properties.

• Melanocytes: Produce melanin, which protects against UV radiation.

• Dendritic (Langerhans) cells: Immune cells that patrol the epidermis.

• Tactile (Merkel) cells: Sensory receptors for touch.

Layers of the Epidermis (from deep to superficial):

1. Stratum basale: Single row of mitotic stem cells; contains melanocytes.

2. Stratum spinosum: Several layers thick; contains keratinocytes, melanosomes, and dendritic cells.

3. Stratum granulosum: 4–6 layers; keratinization begins, cells flatten, organelles disintegrate.

4. Stratum lucidum: Only in thick skin (palms, soles); thin, clear layer of dead keratinocytes.

5. Stratum corneum: 20–30 layers of dead, keratinized cells; provides barrier function.

The Dermis

The dermis is a strong, flexible connective tissue layer containing fibroblasts, macrophages, mast cells, and white blood cells. It houses nerves, blood vessels, lymphatics, hair follicles, and glands. The dermis has two layers:

• Papillary dermis: Thin, superficial areolar connective tissue with dermal papillae (capillary loops, nerve endings, and touch receptors).

• Reticular dermis: Thick, dense irregular connective tissue with collagen and elastic fibers, providing strength and elasticity.

Dermal Modifications and Skin Markings

• Friction ridges: Found in thick skin (fingertips, palms, soles); enhance grip and create fingerprints.

• Cleavage (tension) lines: Parallel bundles of collagen fibers; important for surgical incisions.

• Flexure lines: Dermal folds at joints (palms, fingers, soles).

• Striae (stretch marks): Dermal tears from extreme stretching.

Skin Color

Pigments Determining Skin Color

• Melanin: Produced by melanocytes; protects DNA from UV damage. Varies from reddish-yellow to brown-black.

• Carotene: Yellow-orange pigment, most visible in palms and soles; can be converted to vitamin A.

• Hemoglobin: Pinkish hue in fair skin due to low melanin and visible blood flow.

Clinical Note: Changes in skin color can indicate disease (e.g., cyanosis, jaundice, pallor, erythema, bruising).

Hair

Structure and Function of Hair

Hair (pili) consists of dead, keratinized cells and serves protective and sensory functions. It is absent from palms, soles, lips, nipples, and parts of external genitalia.

• Regions: Root (within scalp, keratinization ongoing) and shaft (above scalp, keratinization complete).

• Hair shaft layers: Medulla (core), cortex (middle), cuticle (outer).

• Pigmentation: Melanocytes in follicles produce hair color; gray/white hair results from decreased melanin and air bubbles.

Hair Follicle Structure

• Hair follicle: Extends from epidermis to dermis; contains hair bulb, root hair plexus (sensory), and wall (connective tissue and epithelial sheaths).

• Hair papilla: Dermal tissue with capillaries for hair nutrition.

• Hair matrix: Actively dividing cells producing hair.

• Arrector pili: Smooth muscle causing "goose bumps."

Types and Growth of Hair

• Vellus hair: Fine, pale body hair of children and adult females.

• Terminal hair: Coarse, long hair (scalp, eyebrows, axillary, pubic regions, face/neck in males).

• Growth cycles: Alternating active and resting phases; affected by nutrition and hormones.

Clinical Note: Hirsutism (excessive hair in females), male pattern baldness, telogen effluvium (stress-induced hair loss).

Nails

Structure and Function of Nails

Nails are scale-like modifications of the epidermis containing hard keratin. They protect the distal dorsal surface of fingers and toes.

• Parts: Root (embedded), nail plate/body (visible), free edge.

• Nail bed: Epidermis under nail plate.

• Nail matrix: Growth area.

• Nail folds: Overlap nail borders; cuticle (eponychium) is the proximal fold.

• Lunule: White crescent due to thickened matrix.

Clinical Note: Nail color/shape can indicate disease (e.g., koilonychia for iron deficiency, Beau's lines for severe illness).

Glands of the Skin

Sweat (Sudoriferous) Glands

• Eccrine (merocrine) glands: Most numerous, abundant on palms, soles, forehead; function in thermoregulation; secrete watery sweat (99% water, salts, wastes).

• Apocrine glands: Axillary and anogenital areas; secrete viscous, milky sweat with proteins and fatty substances; begin at puberty; may act as scent glands.

• Modified apocrine glands: Ceruminous (earwax) and mammary (milk) glands.

Sebaceous (Oil) Glands

• Location: Everywhere except palms and soles; usually associated with hair follicles.

• Secretion: Sebum (oily, bactericidal, softens hair and skin); holocrine secretion.

• Activity: Inactive until puberty; stimulated by androgens.

Summary Table: Cutaneous Glands

Clinical Note: Acne (infected sebaceous glands), cradle cap (seborrhea in infants).

Functions of the Skin

Protection

• Chemical barriers: Sweat, sebum, acid mantle, melanin.

• Physical barriers: Keratinized cells, glycolipids; block water and most substances.

• Biological barriers: Dendritic cells (epidermis), macrophages (dermis).

Body Temperature Regulation

• Insensible perspiration: 500 ml/day under normal conditions.

• Sensible perspiration: Up to 12 L/day when overheated.

• Vasoconstriction: Reduces heat loss in cold environments.

Cutaneous Sensations

• Touch: Meissner's corpuscles, Merkel cells.

• Pressure: Pacinian corpuscles.

• Pain: Free nerve endings.

• Hair movement: Hair follicle receptors.

Metabolic Functions

• Synthesizes vitamin D for calcium absorption.

• Detoxifies some chemicals, activates hormones, produces collagenase.

Blood Reservoir and Excretion

• Holds up to 5% of blood volume; can constrict to redirect blood.

• Excretes nitrogenous wastes, salt, and water via sweat.

Skin Disorders: Cancer and Burns

Skin Cancer

• Basal cell carcinoma: Most common, least malignant; arises from stratum basale.

• Squamous cell carcinoma: Second most common; arises from keratinocytes of stratum spinosum; can metastasize.

• Melanoma: Most dangerous; cancer of melanocytes; highly metastatic and resistant to treatment.

ABCD(E) Rule for Early Detection: Asymmetry, Border irregularity, Color variation, Diameter >6mm, Evolution.

Burns

• Definition: Tissue damage from heat, chemicals, electricity, or radiation; causes protein denaturation and cell death.

• Immediate threat: Dehydration and electrolyte imbalance.

• Rule of Nines: Used to estimate burn area and fluid loss.

• Burn severity:

  • First-degree: Epidermal damage only (redness, pain).

  • Second-degree: Epidermal and upper dermal damage (blisters).

  • Third-degree: Full-thickness; entire skin destroyed (gray-white, cherry red, or blackened; not painful due to nerve destruction).

• Critical burns: >25% body with second-degree, >10% with third-degree, or third-degree on face/hands/feet.

• Treatment: Debridement, antibiotics, temporary covering, skin grafts.

Developmental Aspects of the Integumentary System

Embryonic to Adult Development

• Embryo: Epidermis from ectoderm, dermis and hypodermis from mesoderm.

• Fetus: Lanugo coat (fine hair) and vernix caseosa (protective substance).

• Infancy to adulthood: Skin thickens, glands become active, optimal appearance in 20s–30s, aging leads to thinning, dryness, wrinkles, and increased cancer risk.

Prevention of Aging: UV protection is key (UVA = aging rays, UVB = burning rays).